Metrohm 838 Advanced Sample Processor Installation Instructions - Manual (Page 74)

6.1 Filtration
68 Metrohm 838 Advanced Sample Processor, Annex
6.1.2 Filter working life
A reduction in the recovery rate when analyzing standard solutions can
be used as a possible indicator for the early recognition of an impend-
ing blockage. Ideally these solutions should be made up in the sample
matrix being analyzed.
This means that if a large number of samples are to be analyzed then it
is advisable to measure standard solutions at regular intervals, for ex-
ample after every 5th or 10th sample for sample with a high particle
load. However, it is not possible to make any general prediction about
the number of possible filtration processes. The development of the re-
covery rate with the number of samples may also be subject to large
variations. While with one sample matrix the recovery rate may remain
stable rate for many samples and then drop sharply, with a different
sample matrix it may diminish slowly and continuously.
The time when the filter membrane should be replaced ultimately de-
pends on the sample matrix and the specifications of the analytical
method used. In our experience very fine and suspended particles in
the sample matrix will block the filter membrane more rapidly than
coarse particles, which tend to be carried past the membrane in the
sample flow.
Instructions for replacing the filter membrane are given in Section .
4.2.2.
6.1.3 Filter membrane selection
You can apply existing sample preparation rules to filtration with the
Metrohm 6.2729.110 Ultrafiltration Cell. If you wish to use a different fil-
ter membrane from that supplied as standard then please note that,
even if the particle size is known, the selection of a membrane with an
appropriate pore size does not automatically produce the required re-
sults.
Some investigations have shown that the retention capability of normal
filter membranes does not always correspond to their specified pore
size. The following table shows the qualitative filtering effect of filter
membranes with different nominal pore sizes. The tests were carried
out on aqueous solutions containing silica particles with particles sizes
1.5 μm and 5 μm.